In control engineering, an error signal is derived in response to a measurement of a parameter it is desired to control. An automatic control signal is then derived from the error signal by a controller and used to control some aspect of the system to compensate the error. In its simplest form, this control signal is made to be Proportional (P) to the error signal. However, quite often components of the error signal are also produced.
These are the first order Derivative (D) and Integral (I) of the error signal. Controllers which employ these components are usually called Proportional plus Derivative (P+D) and Proportional plus Integral (P+I) controllers. It is also possible to have a three term controller, i.e. Proportional plus Integral plus Derivative (P+I+D or PID) controller.
The derivative component is used if it is desired to reduce the lag (increase the response time of the system). Integral control is used primarily to eliminate offset, i.e. failure of the controlled parameter to return to the exact set point.
In the implementation of such an automatic control system, the process is normally put on manual control before it is transferred to automatic control. During such a transfer, Integral action plays an indispensable part for a bump-less transfer, but for Proportional only controllers there is no Integral action term for such support.
Some may be of the view that Proportional plus Integral control may be used whenever bum-less transfer is required, instead of employing Proportional action only control, and so there is no real necessity for Proportional action only controllers. However, this is not so, as varying dead-times, which are often caused by sticky control valves, have a deleterious effect on control systems. Hence, as Integral action increases the lag in a system, the preferred choice is Proportional only control for level controls, unless continued deviation from the set-point is totally unacceptable. Moreover, one has to be extremely wary of the Integral action time set, as any unsuitability will not be tolerated in level controls as in flow, pressure, and temperature, since it leads to overflows. To alleviate this, control engineers sometimes even double the gain. Further, it should be noted that vessels that require proportional action only control, often serve merely as intermediaries or buffers. So, having integral action in them impedes their objective. Hence, for level controls, Proportional action only control is often the norm. So, in summary, because there is no integral action to secure a bump-less transfer for Proportional action controllers that deliberately avoid using the integral action term, a novel term or method is required to secure a bump-less transfer, which is what this invention provides.